Arsenopyrite

Arsenopyrite is an iron arsenic sulfide (FeAsS). It is a hard (Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1.[1] When dissolved in nitric acid, it releases elemental sulfur. When arsenopyrite is heated, it produces poisonous sulfur and arsenic fumes which can be fatal if inhaled in large quantities. With 46% arsenic content, arsenopyrite, along with orpiment, is a principal ore of arsenic. When deposits of arsenopyrite become exposed to the atmosphere, the mineral will slowly oxidize, converting the arsenopyrite into an iron arsenate, a relatively stable compound. Arsenopyrite is generally an acid consuming sulfide mineral unlike iron pyrite which can lead to acid mine drainage.

Arsenopyrite
General
CategorySulfide mineral
Formula
(repeating unit)		FeAsS
Strunz classification2.EB.20
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/c
Unit cella = 5.744, b = 5.675
c = 5.785 [Å]; β = 112.3°; Z = 4
Identification
ColorSteel grey to silver white
Crystal habitAcicular, off-square prismatic, stubby; striated; also compact, granular, columnar
TwinningCommon on {100} and {001}, contact/penetration twinning on {101}
Cleavage110 (distinct)
FractureSubconchoidal to rough
TenacityBrittle
Mohs scale hardness5.5 - 6
LusterMetallic
StreakBlack
DiaphaneityOpaque
Specific gravity5.9 - 6.2
Optical propertiesAnisotropism - strong red-violet
PleochroismWeak, white or bluish tint, faint reddish yellow
FusibilityYes
SolubilityNitric acid
Other characteristicsGarlic odor when struck, greenish tinge when weathered, green staining of wall rocks
References[1][2][3]

The crystal habit, hardness, density, and garlic odor when struck are diagnostic. Arsenopyrite in older literature may be referred to as mispickel, a name of German origin.[4]

Arsenopyrite also can be associated with significant amounts of gold. Consequently, it serves as an indicator of gold bearing reefs. Many arsenopyrite gold ores are refractory, i.e. the gold is not easily cyanide leached from the mineral matrix.

Arsenopyrite is found in high temperature hydrothermal veins, in pegmatites, and in areas of contact metamorphism or metasomatism.

Crystallography
Arsenopyrite crystal from the Yaogangxian Mine, Hunan, China (size: 2.7 x 2.0 x 1.7 cm)

Arsenopyrite crystallizes in the monoclinic crystal system and often shows prismatic crystal or columnar forms with striations and twinning common. Arsenopyrite may be referred to in older references as orthorhombic, but it has been shown to be monoclinic. In terms of its atomic structure, each Fe center is linked to three As atoms and three S atoms. The material can be described as Fe3+ with the diatomic trianion AsS3−. The connectivity of the atoms is more similar to that in marcasite than pyrite. The ion description is imperfect because the material is semiconducting and the Fe-As and Fe-S bonds are highly covalent.[5]

Microscopic picture of Arsenopyrite

Various transition group metals can substitute for iron in arsenopyrite. The arsenopyrite group includes the following rare minerals:

  • Clinosafflorite: (Co,Fe,Ni)AsS
  • Gudmundite: FeSbS
  • Glaucodot or alloclasite: (Fe,Co)AsS or (Co,Fe)AsS
  • Iridarsenite: (Ir,Ru)AsS
  • Osarsite or ruarsite: (Os,Ru)AsS or (Ru,Os)AsS

See also

References
  1. Hurlbut, C. S.; Klein, C., 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
  2. http://rruff.geo.arizona.edu/doclib/hom/arsenopyrite.pdf Handbook of Mineralogy
  3. Mindat.org
  4. Mindat Mispickel
  5. Vaugn, D. J.; Craig, J. R. Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. ISBN 0-521-21489-0.
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